Vol.5, No.7A, 15-20 (2013) Natural Science
Inter-center variation in the efficiency of sperm
DNA damage reduction following density
gradient centrifugation
Carmen López-Fernández1*, Stephen D. Johnston2, Altea Gosálbez1, Jose Luís Fernández3,
Juan G. Álvarez4, Jaime Gosálvez1
1Department of Biology, Genetics Unit, University Autónoma of Madrid, Madrid, Spain;
*Corresponding Author: kcchou@gordonlifescience.org
2School of Agriculture and Food Science, The University of Queensland, Gatton, Australia
3Laboratorio de Genética Molecular y Radiobiología, Centro Oncológico de Galicia, A Coruña, Spain
4Centro ANDROGEN, A Coruña, Spain
Received 23 May 2013; revised 24 June 2013; accepted 2 July 2013
Copyright © 2013 Carmen López-Fernández et al. This is an open access article distributed under the Creative Commons Attribution
License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.
This was a prospective multicenter study aiming
at comparing the efficiency of sperm selection
by density gradient centrifugation (DGC) in re-
ducing sperm DNA fragmentation (SDF) in dif-
ferent ART centers. The study was designed
using 290 semen samples collected from 10
different ART centers performing artificial in-
semination, in vitro fertilization and blind as-
sessment of SDF at the University facilities. The
result s sho wed that while there was a significant
reduction in the SDF levels in sperm isolated
from the gradient centrifuged pellet (DGC) com-
pared to neat semen samples (NSS), there was
also significant inter-center variability in the ef-
ficiency to reduce SDF values by DGC (78.5% to
29.2%). Surprisingly, for some patients, the level
of SDF actually increased following sperm se-
lection. The main conclusions derived from this
study were that 1) isolation of sperm from the
gradient pellet by DGC must be performed using
validated, optimized protocols; 2) routine com-
parison of SDF values in NSS semen and in
processed sperm after DGC or swim-up must be
recommended as part of the internal quality
control (QC) of ART laboratories to test the effi-
cacy of sperm processing; and 3) SDF values in
processed spermatozoa should be obtained to
compare with the pregnancy rate when insemi-
nation or fertilization is about to be performed,
otherwise, attempts to predict pregnancy out-
come from SDF could be biased or are essen-
tially meaningless.
Keywords: Assisted Repro ductive Tech nology;
Fertility; Sperm DNA Fragmentation; Male Factor;
Semen Processing
Predicting the success rate of human pregnancy using
estimates of sperm DNA damage to date has been a very
controversial issue, and that is no doubt further compli-
cated by a plethora and permutation of techniques and
procedures that vary in competency and delivery from
laboratory to laboratory, all of which result in a funda-
mental lack of standardization. Figure 1 illustrates some
of the potential factors impacting on the success of SDF
as a predictive measure and its relative interconnected-
ness with other assisted reproduction parameters. These
variables include 1) the type of fertilization technique
Figure 1. Factors affecting the reliability of sperm DNA frag-
mentation as a predictor of pregnancy outcome.
Copyright © 2013 SciRes. OPEN ACC ESS
C. López-Fernández et al. / Natural Science 5 (2013) 15-20
(e.g. natural intercourse, intrauterine insemination [IUI],
in vitro fertilization [IVF] and intra cytoplasmic sperm
injection (ICSI) [1-11]); 2) the use of neat or selected
spermatozoa [4,12]; 3) the specific techniques used for
sperm selection such as swim-up, density gradient cen-
trifugation (DGC) whether used alone or in combination
with intracytoplasmic morphologically-selected sperm
injection (IMSI), Physiological ICSI (PICSI) or Mag-
netic activated cell sorting (MACS) [13-16]; 4) iatro-
genic induced sperm damage during sperm ex vivo han-
dling [17,18]; 5) the type of technique used to assess
DNA damage [19]; and 6) the quality of the donor oocyte
Given the wide range of variables that contribute to
the success of an ART outcome, it is therefore not sur-
prising that this variability is also reflected in the ability
of sperm DNA fragmentation (SDF) to predict pregnancy
outcome. This concept is well illustrated if we consider
just one of the variables in Figure 1, such as sperm se-
lection. In order to improve the predictive power of SDF
it is important that we account and describe the precise
conditions used when conducting our sperm selection
procedure; these issues might include how long the
sperm was stored after ejaculation and prior to sperm
selection, what specific type of sperm selection proce-
dure was employed, for how long and under what condi-
tions was the sperm sample stored or incubated prior to
fertilization and what was the quality of the donor oo-
cytes to be used? In fact, it is logical to assume that the
quality of the oocyte contributes a minimum of 50% to
predictive nature of sperm DNA for pregnancy outcome.
We would, therefore, contend that if SDF is to be used
as a predictive measure of pregnancy success, then there
needs be close attention given to standardizing the spe-
cific processes and conditions of ART that are used in the
different laboratories. Even when such standardization is
accounted for, there is still the potential for innate vari-
ability in the efficiency of SDF reduction usually ob-
served after sperm selection among different individuals
processed within the same laboratory [23]; this pheno-
menon is likely to be related to inherent differences as-
sociated with the ejaculate of each patient, to the techni-
cal competencies of the respective andrologist’s ability to
process the sperm sample or a synergistic combination of
both. In the current study, we explore this general idea by
investigating the relative efficiency of standardized den-
sity gradient centrifugation protocols (DGC), as rou-
tinely used in ART clinics to improve sperm DNA qua-
A total of 290 semen samples from 10 different Span-
ish ART Centers performing artificial insemination and
in vitro fertilization were included in this study. All pa-
tients consented to their participation according Spanish
legislation and confidentiality and following adherence
to Spanish government ethical standards and the internal
ethics committee of the participating clinic. The study
used a double blind strategy for sperm collection and for
sperm analysis. All the semen samples were processed in
each clinic as per “in house” standard protocols. Two
semen samples per ejaculate were prepared for SDF
analysis; 1) Neat semen samples (NSS) were cryopre-
served using standard procedures following immediate
liquefaction of the ejaculate; and 2) Selected semen sam-
ples were processed following liquefaction and density
gradient centrifugation (DGC) at the time equivalent to
that of fertilization and then cryopreserved as per the
NSS to be analyzed at a different facility. Each sample
was processed using the standard methodologies for
DGC as routinely used in each clinic; consequently in
our analysis of results we made no attempt to standardize
reagents or methodologies used for DGC in each center
or methodologies used for cryopreservation. Rather, we
focussed our attention on the final net result of the effi-
ciency to reduce the level of SDF with respect to the
NEAT recovered in each clinic.
Semen samples were collected from case studies of
patients and the decision for patient inclusion was taken
by an andrologist without coordination or reference to
technicians processing the samples. In order to be in-
cluded in the study, the neat semen samples were re-
quired to have a sperm concentration of greater than 20 x
106 per mL and a sperm motility of >50%. Patients with
varicocele, teratozoospermia, oligozoospermia, leukocy-
tospermia were excluded from the study. Cryopreserved
samples from each center were sent to the Unit of Ge-
netics at the University Autónoma de Madrid, where all
the samples were assessed for sperm DNA damage (Ha-
losperm G2-Halotech, Madrid, Spain) by the same tech-
To avoid iatrogenic DNA damage, SDF was immedi-
ately assessed after thawing. NSS and DGC samples
were diluted to achieve a concentration of 10 × 106 per
ml in Modified Ham’s F-10 Basal Medium—HEPES
(Irvine Scientific, CA, USA) and assessed for the level of
SDF. After comparing the values of SDF using Halos-
perm G2 (Halotech, Madrid, Spain) obtained for NSS
and DGC samples, the efficiency in sperm DNA damage
reduction, defined as the e-value (percent reduction of
SDF when NSS and DGC samples are compared) was
SPSS v.15.0 for Windows (SPSS Inc., Chicago, IL,
USA) was used for all statistical analysis. Box-and-
whisker plots were used to show the distribution of the
assessed variable among different ART clinics. Differ-
ences in SDF values among different centers were de-
termined using a Kruskal-Wallis test. The Wilcoxon sign-
ed-rank test for a non-parametric analysis was used to
Copyright © 2013 SciRes. OPEN ACCE SS
C. López-Fernández et al. / Natural Science 5 (2013) 15-20 17
compare two related samples. The Fisher’s Least Sig-
nicant Difference test was used to compare and arrange
differences in the efficiency to reduce SDF using group
means. Correlation analysis was performed using a non-
parametric Pearson test.
Table 1 shows the mean and standard deviation of
NSS and DGC samples and e-value for each ART center.
Figure 2 summarizes the descriptive statistics of SDF
values across the 10 ART centers examined in this study.
There were significant differences in mean SDF values
of both NSS (Kruskal-Wallis; χ2 = 9.7; P = 0.00) and
DGC (Kruskal-Wallis; χ2 = 78.1; P = 0.00) when differ-
ent centers were compared (Figures 2(a) and (b)). When
data for all ART centers was pooled and the efficiency
Table 1. Mean ± (SD) values for sperm DNA fragmentation
(SDF) of neat semen samples (NSS) and density gradient cen-
trifuged (DGC) samples obtained in 10 different ART centers.
The efficiency for SDF reduction—e-value—was expressed as
the percentage of variation obtained for SDF after comparing
NSS and DGC.
Control n NSS DCG Reduction in SDF e-value
1 13 32.2 ± 3.6A 17.3 ± 3.2B 49.6 ± 9.6
2 21 21.8 ± 2.8A 5.2 ± 2.5B 76.0 ± 7.5
3 19 26.8 ± 3.0A 9.7 ± 2.6B 67.1 ± 7.9
4 30 43.9 ± 2.4A 28.8 ± 2.1B 34.1 ± 6.3
5 18 37.1 ± 3.1A 9.7 ± 2.7B 71.6 ± 8.1
6 45 26.3 ± 1.9A 14.8 ± 1.7B 36.2 ± 5.1
7 27 25.5 ± 2.5A 6.8 ± 2.2B 78.5 ± 6.6
8 40 18.5 ± 2.0A 12.7 ± 1.8B 29.1 ± 5.4
9 33 30.0 ± 2.3A 17.1 ± 2.0B 44.8 ± 6.0
10 44 27.1 ± 1.9A 11.3 ± 1.7B 60.6 ± 5.2
Pooled 290 28.9 ± 7.4A 13.3 ± 6.8B 54.8 ± 18.4
Figure 2. Box Whisker plot showing the distribution of sperm
DNA fragmentation (SDF) in (a) neat semen samples (NSS);
and (b) density gradient processed (DGC) semen samples from
different Spanish ART centers (1 - 10); (c) Pooled mean in NSS
and DGC SDF values for all centers.
for SDF calculated, there was a 54.8% reduction in SDF
(e-value = 54.8%) following DGC (Ta b l e 1 and Figure
2(c)). There were significant differences in the efficiency
for SDF reduction between the different centers (Krus-
kal-Wallis; χ2 = 87.1; P = 0.00).
ART centers were then ranked according to their re-
spective e-value (Tab le 2). Center 7 was very successful
in selecting high-quality sperm from the DGC pellet with
an overall reduction efficiency of approximately 80%. At
the other end of the spectrum, Center 6 had a reduction
efficiency of only 29%. Following analysis using the
Fisher’s Least Significant Difference test to compare
group means, significant differences in the efficiency to
reduce SDF were obtained, whereby it was possible to
statistically differentiate 5 different levels of centers
based on their efficiency to reduce SDF (Ta b l e 2 ). This
analysis essentially revealed that 5 clinics were efficient
at reducing SDF, ranging from an e-value of 60% to 80%,
while the remainder presented an e-value ranging be-
tween 30% and 50%.
Although the raw data are not presented, 8% of indi-
vidual patient cases across all the centers actually re-
sulted in an increase of SDF following selection by DGC
when compared to the corresponding neat semen samples.
The lowest efficiency was observed again in Center 6,
where 20% (9/45 cases) of the samples subjected to DGC
yielded higher levels of DNA damage in the DGC pellet
compared to the NSS.
As differences were found in the SDF of the neat se-
men samples from the different clinics, we examined by
means of a correlation analysis, whether the e-value, was
dependent on the initial SDF of the neat semen sample.
This analysis revealed no significant correlation between
these variables (Pearson = 0.01; n = 290; Figure 3(a));
this lack of correlation was also upheld when the data for
Table 2. Efficiency of sperm DNA fragmentation (SDF) re-
ducetion in each center and ranking efficiency per center. Five
different groups (ranked and groups from 1 to 5 in the right
column) were predicted according to the Fisher’s Least
Signicant Difference test; e-value represents SDF reduction
after comparing SDF values obtained in NSS and DGC.
Centre% SDF Reduction (e-value) Mean Group Rank
7 78.5 1
2 76.0 21
5 71.6 321
3 67.1 321
10 60.6 43
1 49.6 54
9 44.8 5
6 36.2 5
4 34.1 5
8 29.1 5
Copyright © 2013 SciRes. OPEN ACC ESS
C. López-Fernández et al. / Natural Science 5 (2013) 15-20
Figure 3. Graphic representation of the correlation between (a)
% sperm DNA fragmentation (SDF) of the neat semen sample
(NSS) and the efficiency of sperm DNA fragmentation (SDF)
reduction and (b) SDF of NSS plotted against SDF in DCG
each center was plotted separately. Collectively, these
findings appear to indicate that the SDF value in the NSS
was not influencing the efficiency of the selection pro-
cedure in reducing SDF in the DGC pellet.
Despite the lack of correlation between the level of
SDF in the NSS and the efficiency for SDF reduction
after DGC, there was still a significant correlation, using
Pearson’s test (Pearson = 0.58; P < 0.01; n = 290), be-
tween SDF observed in the NSS and SDF obtained in
DGC samples (Figure 3(b)), i.e. generally low/high lev-
els of SDF in the NS give rise to low/high SDF following
the DGC procedure.
Two main conclusions emerge of this study 1) in gen-
eral, there was a significant reduction in the levels of
SDF in sperm isolated in the gradient pellet following
DGC; and 2) there was significant inter-center variability
in the efficiency to select DNA intact spermatozoa from
the gradient pellet following DGC.
The reported reduction in sperm DNA damage after
sperm selection is not a novel observation as this effect
has been previously reported in other studies [14,24-26].
Rather, the uniqueness of this study was that it examined
the efficiency to achieve such a reduction across multiple
centers; it was clear from our analysis that the level of
SDF reduction was significantly different across the ART
centers. It is highly likely that this variability in the effi-
ciency of sperm recovery with low levels of DNA dam-
age will have a significant impact on the pregnancy rate
achieved in each of the respective centers, since the
probability of selecting one sperm containing a damaged
DNA molecule is expected to be higher if the efficiency
to remove the damage is low. Our results suggest that it
would be prudent for some centers to develop better
methods of quality control for the sperm selection pro-
In the present study and according to the grouping cri-
teria obtained from the Fisher’s Least Signicant Diffe-
rence test (Ta b l e 2 ), Centers 7, 2, 5 and 3 (as ranked in
Table 2) showed the most efficiency in terms of relative
sperm DNA quality improvement. The e-value for SDF
reduction in this group ranged from 68% - 78% while
those in less efficient group (Centers 8, 4, 6, 9 and 1)
ranged from 30% to 50%. We perceive three plausible
explanations for these observations. Firstly, we suggest
that the inter-center differences are related to the possible
differences in the design and performance of the DGC
protocol, which may be related to centrifugation speed,
centrifugation time and the ability to aspirate a clean
pellet. We also suggest that the skill associated with the
isolation of the different density gradient layers may also
be a significant factor contributing to this variability,
because layer mixing and contamination may occur dur-
ing aspiration of fractions 1 and 2, where the highest
levels of immature spermatozoa and the higher levels of
affected sperm are typically found [27]. This is likely to
be the most parsimonious explanation and would pos-
sibly account for those cases where the level of SDF af-
ter DGC was actually higher than that found in the NSS.
Secondly, even though there were relatively strict in-
clusion parameters set for this study, the quality of the
individual semen samples e.g., percentage of ROS-pro-
ducing immature spermatozoa [28,29] is also likely to be
an additional factor contributing to the observed in-
ter-center variability, especially since the initial SDF
values obtained in neat semen would be significantly
magnified following processing and this could be easily
linked to an accumulative presence of sperm damage at
different levels [23,30]. Either independently or syner-
gistically, these factors could act to produce subtle dif-
ferences that influence sperm DNA survival.
Copyright © 2013 SciRes. OPEN ACCE SS
C. López-Fernández et al. / Natural Science 5 (2013) 15-20 19
The other potential explanation for the observed vari-
ability between clinics with respect to the efficacy in the
reduction of spermatozoa with DNA damage after DGC
could be the often overlooked negative impact of iatro-
genic damage prior to, during and following sperm se-
lection. Sperm DNA damage tends to increase after eja-
culation and differences in the sperm DNA longevity
exists among different individuals. This phenomenon has
been previously been described [31,32] and appears to be
related to the lag time between sperm selection and DNA
damage analysis. The dilemma as we see it is that the
high efficiency of SDF reduction in some centers (close
to 80%) may in fact only be compensating for the iatro-
genic DNA damage inherent to the DGC protocol, ren-
dering the DGC a potentially redundant process. When
the level of SDF after the DGC protocol is higher than
that of the NSS, as it happens in some of the cases re-
ported in this study or published elsewhere [23], the rea-
lity is that we may be wasting our time and in fact, in
some cases actually reducing the chance of successful
pregnancy by increasing the level of SDF when exposing
sperm to selection procedures.
We derive three main “take-home” messages from this
study: 1) isolation of sperm from the gradient pellet by
DGC must be performed using validated, optimized pro-
tocols; this should reduce the observed inter-center varia-
tion, not only in SDF values but also for other seminal
parameters; 2) routine comparison of SDF values in NSS
and DGC or swim-up should be recommended as part of
the internal quality control of the ART laboratory to test
the safety and efficacy of sperm processing; and 3) SDF
values in processed spermatozoa should be obtained to
compare with the pregnancy rate when insemination or
fertilization is about to be performed, otherwise, attempts
to predict pregnancy outcome from SDF could be essen-
tially confusing if not meaningless.
The author(s) declared no potential conflicts of interest with respect
to the research, authorship, and/or publication of this article. This re-
search was supported with public funding (Spanish Ministry of Science
and Technology (MCYT: BFU2010-16738).
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